Add new JEXL transformation: hextostring

[mrutid]

hextostring: (val) => new TextDecoder().decode(new Uint8Array(val.match(/.{1,2}/g).map(byte => parseInt(byte, 16))))

So:

"706174617461"|hextostring => "patata"

[fgalan]

Should be also included in the Urbo functions catalogue?

[fgalan]

Fixed by PR #1368